What is mol rxn? Units in thermochemistry calculations.

I pulled the following chemical equation from the CollegeBoard AP® Chemistry Course and Exam Description, Fall 2020:

2SO2(g) + O2(g) → 2SO3(g)    ∆H° =  -198 kJ/mol-rxn


This equation means that:

when two moles of gaseous sulfur dioxide react with one mole of gaseous oxygen in standard conditions of 298 K and atmospheric pressure, the reaction yields two moles of gaseous sulfur trioxide and releases 198 kJ of excess energy.

While the chemical equation itself is standard, the notation for the reaction’s change in enthalpy is nuanced and can be a sore point sowing confusion. Let’s break the enthalpy term down piece by piece:

• H denotes a state of enthalpy.

• ∆H denotes a change in enthalpy and implies that ∆H = Hfinal Hinitial and so ∆H = Hproduct Hreactant.

Because ∆H is negative in this reaction, the products have a lower enthalpy than the reactants; this reaction releases energy; and the released energy would be observed as excess heat emerging from the source. All to say that the reaction is exothermic.

The prime or degree mark in ∆H° denotes standard reaction conditions, such that the reaction is conducted at 298 K and 1 atmosphere of pressure.

∆H° = -198 kJ denotes that the reaction, with the written stoichiometry and conducted at standard reaction conditions, releases exactly 198 kJ of energy.

∆H° = -198 kJ/mol-rxn contains an additional mark “/mol-rxn” that was introduced by the CollegeBoard. This mark has been a source of significant confusion because it is not present in traditional chemistry texts, is thus foreign to most chemists, and was introduced into AP® exam preparation materials without explicit definition or justification.

The “/mol-rxn” mark is especially confusing because in most reactions, the stoichiometry of each reactant and product differs. There is simply no such thing as a reaction mole unless we refer to a mole of reactions, i.e., 6.022E23 reactions occurring in concert, which we are certainly not.

Thus, the only way to interpret the “/mol-rxn” mark is as “the reaction stoichiometry as written”. Thus, “/mol-rxn” simultaneously refers to the heat released “per mole of O2 reactant”, “per 2 moles of SO2 reactant”, and “per 2 moles of SO3 product.” And so, there is no real difference between “∆H = -198 kJ” and “∆H = -198 kJ/mol-rxn“; the two notations mean the same thing, except that the second notation is ripe for controversy and confusion.

My best advice is to teach students to read the “/mol-rxn” mark as “with the reaction stoichiometry as written.” Please ensure students understand that senior chemists will be unfamiliar with this notation, rightly find it unnecessary and arbitrary, and will resist using it in modern chemistry practices. But students who will take the AP® exam must be explicitly taught how to interpret the “/mol-rxn” mark, else they could easily mistake its meaning and come to the wrong answer on the AP® exam, even if they fundamentally understand all relevant chemical concepts involved.

Author: J Matt Jackson, PhD


Burning Money Experiment (Chemistry)

Burning money experiment for chemistry

This discrepant event is short but can be made into a lot of fun with a little theater: Find 
the largest dollar bill amount in the classroom. I like to ask the class who the “big shot” is 
in the class and then proceed to ask them how much money they have on them.

From there, proceed with the demonstration. Students will make observations before
 hypothesizing the science behind what transpired.

Video credit: BeardedScienceGuy

The sheet students fill is very short and simple. I print this as a quarter-sheet front and
 back so that students don’t fret about the hypothesis while they are recording their

What’s nice is that this works for any level of chemistry student. For general chemistry, 
I’d hint to students that the solution was of rubbing alcohol and water. For AP students I
 would give no hints of the solution, and I challenged them to write about what happened at
 the molecular level.

Note: You can adjust the size of the flame by increasing the IPA concentration. I’ve used
 as much as 70:30 IPA: Water by volume.

Setting the tone for lab work:
 You can use this the next day to highlight examples of stellar observations and hypotheses 
to set the table (or work bench I should say) for lab write ups.

Record your observations:


Write your hypothesis for what transpired. Be as specific as possible:

 Do not perform this experiment near anything flammable. Make sure to know the
 procedures thoroughly and have a fire extinguisher nearby before performing this
 experiment. Follow all lab safety rules carefully.

 If you decide to perform this experiment, you do so at your own risk.



Top 3 Things to do returning to the classroom from distance learning

The 2020-2021 school year had its challenges, no doubt. Here are the top three things you should do to ensure a successful year for 2021-2022. Without further ado, let’s jump in!

1) Make your students excited to be back!

Burning dollar note lab

Yes, this is the absolute best thing you can do for your students. Students learn drastically better when they are engaged. A student can be engaged on multiple levels but the very highest level of engagement a student can have is when they are genuinely fascinated with the topic.

Dedicate a chunk of Day 1, if not the entire period to a fun, inquiry-based activity (they can read the syllabus on their own and you can briefly quiz them on it day 2). There are many discrepant events you can do and make a short, easy inquiry activity from. However, a teachers’ old favorite involves taking money from a student and lighting it on fire (click here for more).

An exciting and engaging activity like this will boost your students’ dopamine and serotonin levels in relation to your class. This subsequently will have a motivating and fulfilling effect on students that will carry into the year if you can keep elements of this excitement going. (So yes, your students will learn better when they have a little bit of fun). Throw this in after a year of computer-learning and you may just inspire a few future chemists.

Disclaimer: All lab experiments and science activities have potential hazards. All experiments and activities presented on this website should be used with caution and good judgement. Safety rules for laboratory conduct should always be observed whenever working in a laboratory.

2) Provide structure on Day 1

top 3 things to do on first day of school

This was hard to come by last year (not just for students but all of society). Providing this on Day 1 will help students form a clear idea of what to expect from your course. Students (and people) learn better when there are clear expectations of what and how they should learn.

Some things we advise structuring:

Seating Chart + Lab Partners: AP students know how they learn best, so they can seat themselves, right? Sort of. The biggest driver of students picking their seats is anxiety-reduction. Students will sit next to whomever makes them feel the most comfortable–which often means their friends. Sometimes, friends make good learning partners, but often they do not. Friends distract each other. And, for labs, it often means a reduction in work by one member.

Assigned seating quells the “where-to-sit” anxiety students may have on the first day. Additionally, it shows them that YOU are in charge of the class, and that everything–including their seating arrangements–has a purpose.

Unit Outline: This contains the learning objectives, assignments and assessments for each unit. Brownie points go to you if the learning objectives are correlated with their respective assignments.

Students retain concepts better when they are conscious of what their learning objectives are. This reinforces the metacognitive process. It is strongly recommended to delineate these to students before each unit, and to review them prior to the unit exam. They should also be easily interpretable for students. You may feel that using your own wording is best for some objectives.

These are just a couple things where structure is beneficial. Not every component of the class requires – or benefits for that matter – from structure. In fact, structure very much falls into the Goldilocks principle: too little is entropic, too much is constricting.

3) Have backup

AP chemistry supplementary resources

Not just in case the pandemic closes schools again! It’s great to have an alternative resource for reaching students who are struggling in the course. Nearly 50% of students score either a 1 or 2 on the AP chemistry exam! These are students who need additional support!

A study hall or office hours are great for autonomous students. It would be nice if every struggling AP student sought their own help when they needed it: as you’re probably aware, they don’t, unfortunately. However, a gentle push for such a student can often go a very long way.

We at Viziscience exist to offer this additional help to you and your students. Teachers, we know how much work it is to provide individualized curriculum and instruction for every student. Our modules allow you to assign individual topics–or entire units to your students. You’ll be able to monitor each student’s progress without having to do any grading yourself. This valuable time saved you can spend elsewhere to improve your classroom or lessons.

To find out more, don’t hesitate to reach out to us at contact@viziscience.com or send us a message.


AP chemistry Summer Prep Course

The age-old question for teachers who want to prepare their students for the upcoming school year by providing summer work is:

How can I ensure that my students complete this work?

The simple answer is – you can’t. However, teachers continue to ask this question because having a class of students that are all on the same page in terms of pre-work is ideal and is rarely – if ever – achieved.

Typically, summer work is thought of as reading and book reports. However, for teachers of Advanced Placement (AP®) Chemistry, Viziscience’s Summer Program version of their curriculum resource is truly the ideal pre-coursework.


AP chemistry summer prep course


For teachers looking for ideas on motivating their students to complete the program, here are some ideas:

  • Incentivize: Simply put, if there is no noticeable incentive for a task, students are not likely to complete it. A noticeable incentive is one whose effect is positive and will directly benefit that student. Some possible incentives are providing extra credit for its completion, making the work the first graded assignment of the year, or providing points or assignment passes for work completion that students can then choose how and when to use. It is also appropriate to offer a combination of these incentives. This summer program is high quality and will serve to set up students for success in AP Chemistry by building confidence as well as a knowledge base. Therefore, incentivizing its completion is not only understandable, but is ultimately beneficial for the students.
  • Create and Monitor a Collaboration Tool for Students: Engaging in high-level academic work over the summer on the computer, especially after over a year of often virtual instruction during a global pandemic, can feel extremely isolating for students. If students feel the work is not accessible, they might shut down. However, if students have a place to talk to each other, ask each their peers questions, vent, and collaborate, they are more likely to engage in the work. This can be done via a group chat app on the phone, some sort of social media platform or learning management system. There are apps and tools that allow students to post and respond to one another in one place. Even a simple Google Doc can be used to facilitate communication. The key with this strategy is to monitor the chat to ensure conversation remains productive and safe, and in case a question arises that stumps everybody.
  • Inform and Involve Caretakers: More than the typical elective or even core class, having the support of caretakers at home with AP Chemistry is important. When caretakers are aware of work that needs to be done, best-case scenario, they can ensure it gets done and might even be able to help their scholar complete the work. Worst-case scenario, they do not ensure it gets done – which is a more likely outcome without their involvement. It is important for caretakers to stay informed on AP subjects and tests anyway, so it is never too early for them to start to see how hard you are working as their student’s teacher to make sure their scholar succeeds.

AP Chemistry is difficult and it is intellectually taxing for students. However, with the promise of possible college credit for students who do well on the AP exam, it’s a class that might require a little extra leg work. The bottom line is you can lead a horse to water but you can’t force it to drink. Likewise, you can give students the opportunity but it’s up to them to take it. Contrary to what’s just said, I believe if you spell out the benefits – how this will ultimately prepare them for success and help prevent them from falling behind in the school year, I think most AP students will want to take the opportunity especially seeing their peers doing it.